Indicator fuses, also referred to as alarm fuses, are well known in the art. They are a circuit-protecting type of fuses that, upon tripping or "blowing", give an indication of being in the tripped state. The indication is typically visual (e.g., a change in color or position of an indicator "dot" or the lighting of a lamp or LED) or electrical (e.g., closure of an alarm electrical contact, typically by means of movement of a spring-operated contact-making plunger).
An illustrative example of an indicator fuse is the 70-type fuse of AT&T. The 70-type fuse is widely used to provide external-short protection to telecommunication line circuits. The fuse uses a conventional fusible element that melts and opens the circuit under application of too much current thereto. The fusible element is attached at one end to a capped coil spring and the cap bears a colored bead. When the fuse operates and the fusible element melts, it releases the spring which forces the cap on the spring against an alarm terminal of the fuse block to raise an alarm indication, and also causes the colored bead to protrude from the block to identify the fuse that blew.
Because of the self-destructive nature of the fusible element, the fuse must be replaced when the short that caused the fuse to blow is removed. The replacement of the fuse requires a maintenance visit from a craftsperson. Because of the large numbers of line circuits, and hence the large numbers of 70-type fuses present in typical telecommunication equipment such as concentrators, multiplexers, and switches, maintenance visits for the purpose of fuse replacement constitute a significant portion of all maintenance visits and therefore contribute greatly to the cost of maintaining and servicing the equipment. It is desirable to decrease this cost.
Reusable fuses, that use either manually or automatically-resettable fusing elements, are also known in the art. But resetting of a manually-resettable fusing element requires manipulation by a craftsperson, and therefore does not reduce the need for maintenance visits. While the use of automatically-resettable fusing elements does avoid this problem, it creates a new one: automatically-resettable fusing elements typically do not produce the kinetic energy (e.g., plunger movement) that is often necessary to set and/or reset the indication of the fuse's state. Consequently, a barrier exists to the use of automatically-resettable fusing elements in indicator fuses, at least those of the electrical, alarm contact-closure, type.